Induction oven for melting metals

ABSTRACT

An induction heating device which raises the temperature of a metal to be heated for one of melting or hot machining while providing considerably energy saving, increasing yield and observing current safety standards. The device (10) uses a cavity (11) to receive the metal to be heated and at least two magnetic yokes (13) arranged around a periphery of cavity (11), each yoke supporting an independent induction coil (14). The induction coils are mounted and wound in the same direction such that a north pole, of each coil, is located on one side of the cavity and a south pole is located on an opposite side of the cavity. The inductive coils are arranged so as to generate active non null magnetic field zones and inactive zones of null magnetic fields distributed about the periphery of the cavity. An inactive zone of null magnetic fields is located between each adjacent active non null magnetic field zone. The induced current is self-enclosed thereby producing high heating power and the invention is applicable to melting, forging, reheating, transforming, and working metals by induction.

FIELD OF THE INVENTION

The present invention relates to an induction heating device to raisethe temperature of metals with a view to melting or hot working them,said device comprising at least one cavity defined by a ladle designedto receive the metals to be brought up to a temperature greater than orequal to their melting point or by an oven designed to receive thebillets of metal to be brought up to a temperature which is lower thantheir melting point, this temperature being determined to forge themetals, along with induction heating means for said ladle or said oven.

BACKGROUND OF THE INVENTION

Induction heating devices are well known in the field of metal melting,forging billets of metal with a view to hot machining them, metal oralloy working or smelting. Nevertheless, in known devices, the inductioncoil(s) are wound around the cavity receiving the metal and are usuallycooled by a water-cooling circuit. There is a possible risk of leaks inthe cooling circuit, which is totally prohibited when working withmolten metals. Furthermore, the efficiency achieved with thisconfiguration generally does not exceed 40 to 60%. This efficiency isproportional to the ratio of the inductor's surface area and the surfacearea of the stack. What is more, the magnetic field created by theinduction coils is an open field. Consequently, the losses aresignificant and amount to around 1/3 of the total power applied.

In this field of application, the main technical constraints to be takeninto account are as follows:

protecting people from electromagnetic fields, as laid down by Frenchstandards and European directives (CENELEC and DG5),

efficiency, and

safety (it is essential that any contact between the water and themolten metal be avoided).

Other induction heating devices have attempted to provide a solution tothe first problem posed. Some devices are described in the publicationsDE-C-266 566, US-A-1 834 725 and BE-A-351 671 and comprise at least twoyokes arranged around the cavity receiving the metal to be heated, whichare L-shaped or C-shaped, so that the ends converge toward the inside ofsaid cavity. Each yoke bears an electric coil creating a magnetic fieldwhich closes through said cavity. An improvement to this type ofconstruction is described in the publication DE-C-277 870 which providesfor three yokes, the coils of which are fed individually andphase-shifted in order to create a rotary field. In all theseembodiments, all the magnetic fields are radial, which means that thelines of electric flux cross the cavity's axis and cross right throughit axially. These magnetic fields create an induction current limited tothe periphery of said cavity and generate an increase in the temperatureof the metal in this zone with the remainder of the metal being heatedby conduction. The efficiency of these various devices and even the oneproviding for a rotary field, remains very low, as the effective part ofthe field using for heating purposes is small.

SUMMARY OF THE INVENTION

The present invention proposes to overcome the drawbacks of the priorart and meet the requirements of current standards by means of aninduction heating device which makes it possible to achieve efficiencyin the region of 80 to 95%, with smaller induction boxes, a higher powerfactor (cos φ0.8 instead of 0.05 or 0.1) and requiring less electricenergy consumption. Furthermore, the present invention makes it possibleto speed up the temperature rise and therefore the melting or hotmachining of the metal, thus also favoring energy savings. The energysavings achieved by the present invention are such that a return oninvestment within about two years can be envisaged, which is veryappreciable in commercial terms.

The aim is achieved by a device such as the one described in thepreamble and characterized in that the induction coils are fitted in thesame direction so that their north pole is located on one side of thecavity and their south pole on the opposite side, in that they arearranged so as to generate null magnetic field zones arrangedalternately between non null field zones spread out on the periphery ofthe cavity, the non null field zones each comprising a maximum fieldzone associated with two decreasing field gradient zones arranged oneither side of said maximum field zone, extending as far as theneighboring null field zones, as well as a null field zone located inthe center of this cavity, the non null field zones forming active heating zones separated by said null field zones forming inactive zones.

Each yoke offers the advantage of comprising an elongated branchextending from one end of the cavity to the other, which is arrangedsubstantially parallel to the axis of this cavity and bears at least oneinduction coil designed to generate one of said active heating zones.

In a first form of embodiment of the invention, each yoke shows anL-shaped profile and comprises said elongated branch and a lateralbranch extending substantially perpendicular to said elongated branchand substantially radially in relation to the end of the cavity.

Said cavity may be a ladle, said lateral branch extending radially inrelation to the bottom of this ladle in the direction of its center.

In a second form of embodiment of the invention, each yoke shows aU-shaped profile and comprises said central elongated branch and twolateral branches extending substantially perpendicular to said centralelongated branch and substantially radially in relation to the two endsof the cavity.

In this version, the cavity is preferably an oven and at least one ofsaid lateral branches extends as far as the vicinity of the longitudinalwall delimiting said cavity.

In a third form of embodiment, each yoke shows a C-shaped profile andcomprises said central elongated branch and two lateral branchesextending substantially perpendicular to said central elongated branchand substantially radially in relation to the two ends of the cavity.

In this version, at least one of said lateral branches extends as far asthe vicinity of the lateral wall delimiting said cavity.

Said cavity may be a ladle, with one of the lateral branches extendingradially in relation to the bottom of this ladle and the other lateralbranch being a free section directly attached to a cover designed toclose said ladle and extending radially in relation to this cover as faras the vicinity of the lateral wall delimiting said cavity.

In a fourth form of embodiment, each yoke is I-shaped and comprises saidelongated branch and two lateral branches extending substantiallyperpendicular to said elongated branch and substantially radially inrelation to the two ends of the cavity.

In this version, at least one of said lateral branches extends radiallyas far as the vicinity of the lateral wall delimiting said cavity.

Each of said coils preferably extends substantially over the wholelength of the yoke's elongated branch.

The heating means favorably comprise a number n of yokes spread out atregular intervals on the cavity's periphery.

Depending on the type of heating being sought the coils can be fedindividually by an alternating electric current and this power supplycan be phase-shifted from one coil to another. This power supply shiftfrom one coil to another can be determined by an arithmeticalprogression.

BRIEF DESCRIPTION OF THE DRAWINGS

The various coils can also be fed by several generators designed tocreate a rotary field.

The present invention and its advantages shall be further disclosed inthe following description of two examples of embodiment, with referenceto the attached drawings, in which:

FIG. 1 is an axial cutaway view of a device according to the inventionintended for metal melting purposes,

FIG. 2 is a cutaway topview along the II-II arrows of the oven in FIG.1,

FIGS. 3 and 4 are perspectives of the device in FIG. 1, topview andbottom view respectively.

FIG. 5 is a longitudinal cutaway view of a device according to theinvention intended for metal forging purposes,

FIGS. 6A and 6B are diagrams representing the lines of the magneticfield of the device according to the invention and a standard devicerespectively,

FIGS. 7A and 7B represent the cavity seen from above and schematicallythe flow of the induced currents for the device according to theinvention and a standard device respectively, and

FIGS. 8A and 8B are diagrams representing the distribution of the heatpower for the device according to the invention and a standard devicerespectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 1 to 4, the induction heating device 10comprises a cavity 11 formed by a ladle 12 generally made of arefractory material, which is designed to receive the metal to bemelted, along with induction heating means designed to raise thetemperature of the metal by means of a magnetic flux until it melts.

These heating means comprise independent magnetic yokes 13 arrangedaround the ladle 12 a sufficient distance d away to allow inductioncoils 14 to be put in place. Each yoke 13 is L-shaped overall andcomprises an elongated branch 13a which is substantially parallel to theladle's 12 axis and extends substantially over the whole height of saidladle 12 as well as a lateral branch 13b which is perpendicular to theelongated branch 13a and extends radially in the direction of the bottomof said ladle 12. The ends of the branches 13a and 13b are curved sothat they are as close as possible to the ladle's 12 wall.

Depending on the case, the yokes 13 can be either C-shaped or I-shapedor even only have said elongated branch 13a. In any case, the elongatedbranch 13a of the magnetic yokes 13 extends substantially over the wholeheight of the ladle 12 and the lateral branch is oriented radially andpreferably extends as far as the vicinity of the ladle's 12 wall. Insome cases, the lateral branches can constitute a free section arrangedperpendicular in relation to the elongated branch which folds forexample in the vicinity of the bottom of the ladle if these branches areattached under a cover designed to close the ladle. In other cases, thelateral branches can extend as far as the middle of the bottom of theladle. Finally, these lateral branches can be profiled so that theypartly cover the surface area of the bottom of the ladle. In any case,the two lateral branches 13b of the same yoke must not both extend asfar as the center of the ladle 12, at least one of them must stop in thevicinity of the edge of this ladle.

The number of these magnetic yokes 13 is n equal to eight (in FIGS. 1and 2) and six (in FIGS. 3 and 4) and they are arranged normally at thesame distance from one another around the ladle 12. This number n is notrestrictive. It can also be lower or higher, even or odd, depending onthe type of ladle and its specifications: capacity in tons of metal,heat power, etc.

The unit described above and formed by the ladle 12, the magnetic yokes13 and the induction coils 14. is housed in a tank 15 designed forprotection and insulation purposes which can be provided with a cover ora door (not shown), this tank being mounted tilting on a chassis or abracket (not shown) around a joint pin 16 passing through two lugs 17securely fixed to said tank 15. During the melting operation, this tank15 can be hermetically sealed or not and can be placed under a vacuum tooptimize the operation of the induction heating means. After the metalhas been melted, the cover or the door opens, the tank 15 tilts aroundits joint 16 to empty the ladle 12 of its molten metal content intomolds for example in the same manner as in the devices of the prior art.

Each induction coil 14 is arranged around the elongated branch 13a ofeach magnetic yoke 13 and extends substantially over its whole length.These induction coils 14 are fed individually with an alternatingcurrent and generate a magnetic flux, whose lines of electric flux areshown in FIG. 6A. Due to the magnetic yokes 13, this magnetic flux ischanneled, directed and closed in a peripheral zone inside the ladle 12in the vicinity of said yoke, through the metal to be heated. Only asmall part of the flux passes outside. Losses are therefore small. FIG.6B shows the lines of electric flux for a device of the prior art whichis not equipped with a magnetic yoke and highlights very clearly theimprovement in the concentration of the lines of electric flux aroundthe ladle 12 achieved using the device according to the invention withreference to FIG. 6A.

Furthermore, in the device according to the present invention, the coils14 are all oriented in the same direction, their north pole beinglocated on one side of the yokes and their south pole on the other side.The poles of the same kind thus repel each other by repelling theirrespective magnetic fields, thereby creating null magnetic field zones40 alternating with non null magnetic field zones 41, shownschematically on FIG. 7A. Therefore, the non null field zones arecentered on the radial planes 42 passing via the yokes' 13 axes andextend on either side of a maximum field zone, in the vicinity of theperiphery of the cavity 11. These non null field zones 41 thereforecomprise a maximum central field zone and two decreasing field gradientzones arranged on either side of the maximum field zone up to theneighboring null field zones 40. The lines of electric flux are arrangedsymmetrically on either side in relation to said radial planes 42passing via the center of the cavity 11 and passing via the yokes' 13axis. The null field zones 40 thus delimit active zones 41, made up ofthe maximum field zones and the decreasing field gradient zones,corresponding to the metal's heating zones. As a result, contrary to thedevice of the prior art, in which the heating zones 51 extend along theperiphery of the ends of the cavity 11 as shown in FIG. 7B, the activeheating zones 41 are delimited on defined angular portions of theperiphery of said cavity 11. In other words, in each active zone 41, themagnetic field induces a current 43 generating a heat power, thiscurrent being obliged to close on itself forming a loop in this activezone, whereas in the devices of the prior art, the induced current 53extends all around the periphery of the cavity. Furthermore, we do knowthat the induced current generates a heat power which is directlyproportional to the volume of metal crossed by said current.Consequently, the fact that the currents 43 induced by the coils 14 arelocated in said active zones 41 makes it possible to significantlyincrease the volume of metal crossed by all the induced currents, incomparison with the volume of metal crossed by the current induced onthe periphery. The result thus achieved is an increase in the volume ofmetal heated for the same induced current and therefore much greaterefficiency.

FIGS. 8A and 8B make it possible to compare the distribution of the heatpower between the devices of the prior art and that of the invention,the white zones representing the highest heat power which fallsgradually in the darker zones. There are of course various temperaturelevels which correspond to these various levels of heat power. Thesefigures are illustrations of real tests carried out for the same inducedcurrent and therefore the same magnetic field generated by each coil. InFIG. 8B, which illustrates the prior art, the white zones correspond tothe heating zones 51 and are limited to the periphery of the ends of thecavity with the inside being totally dark. In FIG. 8A, which illustratesthe present invention, the white zones are spread around thecircumference of the cavity and over its whole length. Several whitezones can be observed, spread over the periphery of the cavity,extending over its whole length and being slightly prolonged toward theinside. These white zones correspond to the active heating zones 41delimited from one another by said null field zones 40. It can theneasily be seen that the entire surface covered by the white zones inFIG. 8A is much larger that the one in FIG. 8B. This increase in surfacetherefore has a direct effect on the efficiency of the induction heatingwhich can reach 80 to 95%.

Furthermore, the distance d which separates the elongated branch 13a ofthe magnetic yoke 13 supporting the induction coil 14 from the ladle 12may be relatively large to make it possible to increase the thickness ofthe refractory walls of the ladle 12 and limit heat losses. Furthermore,induction coils 14 with smaller diameters, lower outputs and greaterpower factors than those of the prior art can be used. As a result, theJoule's heat losses are also limited and the induction coils 14 do notneed to be cooled by a specific water circulation. Air ventilation issufficient to ensure cooling of said coils.

Under the effect of the active heating zones 41, the temperature of themetal rises quicker in certain zones, thus causing a shift or anautomatic stirring between the hot masses of metal and the cooler onesso that in turn their temperature also rises to obtain a homogeneousmolten mixture.

This stirring is considerably improved and accelerated by individuallyfeeding the to induction coils 14 with a shift in the power supply fromone coil to the next and so on, in a clockwise or anticlockwisedirection. This phase shift in the power supply generates acircumferential and helicoidal stirring of the metal inside the ladle12. The direct consequence of this form of stirring is a quickerhomogenization of the temperature gradient in the metal, making itpossible to considerably shorten the time required for it to soften andmelt, thereby leading to significant energy savings. This forcedstirring can also be achieved by feeding each coil via an independentgenerator. All the generators can then be synchronized so as to obtain arotary field, thus creating the effect of a helix in the molten metal.

FIG. 5 illustrates an alternative embodiment of the invention in whichthe device 30 comprises a cavity 31 formed by a oven 32 generally madeof a refractory material and designed to receive billets of metal 35 tobe hot machined, along with induction heating means 33, 34 designed toincrease the temperature of said billets to a temperature lower thantheir melting point, by a magnetic flux. These heating means comprise,as in the previous example, independent magnetic yokes 33 arrangedlongitudinally around the oven 32 and a sufficient distance d to houseinduction coils 34 there. Each yoke 33 comprises an central elongatedbranch 33a and at least one lateral branch 33b, 33c perpendicular to thecentral elongated branch 33a. The central elongated branch 33a of themagnetic yokes 33 extends substantially over the whole length of theoven 32 and the two end branches 33b and 33c extend radially as far asthe vicinity of the oven 32. In the example shown, the yokes 33 aregenerally U-shaped. Each induction coil 34 is arranged around thecentral elongated branch 33a of each magnetic yoke 33 and extendssubstantially over its whole length.

The number of magnetic yokes 33 and induction coils 34, they way theyoperate and their advantages are identical to the ones describedpreviously. Likewise, it is also possible to optimize the homogenizationof the temperature gradient inside and right along the oven 32 byfeeding the induction coils 34 with a phase shift from one coil to thenext or by independent synchronized generators.

It clearly emerges from this description that the invention reaches theintended aims. Its primary advantage is of course the energy savingswhich this induction heating device makes it possible to achieve whilecomplying with current safety standards. Consequently, even if thisdevice requires a greater overall investment compared with a knownstandard device, the energy gains achieved make it possible to envisagea return on investment within around two years.

The present invention is not limited to the examples of embodimentdescribed but can be widened to include any modification and alternativewhich is obvious for the expert. As specified, the number of magneticyokes and induction coils is not restricted. Likewise, the shape of themagnetic yokes may vary according to the ladle or the oven. The yokesmay also be made up of several free sections. Managing the coils' powersupply may also be deferred.

What is claimed is:
 1. An induction heating device (10, 30), for heatinga metal, comprising:a ladle (12) defining at least one cavity (11) forreceiving and heating a metal, via induction heating devices (13, 14),to a temperature at least equal to a melting temperature of the metal,the induction heating devices (13, 14) comprising at least two magneticyokes (13) arranged around a periphery of the cavity (11) and having alongitudinal length which is greater than a height of the cavity (11);each yoke having at least one independent induction coil (14), and eachinduction coil (14) is wound in the same direction so that a north pole,of each one of the induction coils (14), is located at one of a top andbottom end of the cavity (11) and a south pole, of each one of theinduction coils (14), is located an opposite end of the cavity (11),each one of the induction heating devices (13, 14) generating, about theperiphery of the cavity, a non null field zone (41) and a null magneticfield zone (40) being created between each adjacent pair of non nullfield zones (41); each non null field zone comprising a centrallylocated maximum field zone and a decreasing field gradient zone arrangedon either side of the central maximum field zone with each decreasingfield gradient separating the central maximum field zone from one of thenull field zones (40); and a central null field zone (40) located in thecenter of the cavity, and each one of the non null field zones formingan active heating zone and each one of the null field zones forming aninactive heating zone.
 2. The induction heating device according toclaim 1, wherein each of the at least two yokes (13) further comprise anelongate branch (13a) which extends from adjacent a top end of thecavity to adjacent an opposed bottom end of the cavity, and the elongatebranch extends substantially parallel to a longitudinal axis of thecavity (11) and supports a t least one induction coil (14) forgenerating one of the active heating zones (41).
 3. The inductionheating device according to claim 1, wherein each of the at least twoyokes (13) has an L-shaped profile and comprises an elongate branch(13a), which extends substantially parallel to a longitudinal axis ofthe cavity (11), and a lateral branch (13b) which extends substantiallyperpendicular to the elongate branch (13a) and substantially radiallyinward in relation to the longitudinal axis of the cavity (11).
 4. Theinduction heating device according to claim 3, wherein the lateralbranch (13b) extends radially toward the longitudinal axis of the ladleadjacent a bottom surface of the ladle.
 5. The induction heating deviceaccording to claim 1, wherein each yoke has a C-shaped profile andcomprises an elongate branch (13a), which extends substantially parallelto a longitudinal axis of the cavity (11), and two lateral branches(13b, 13c), a first one of the two lateral branches (13b) extends from afirst end of the elongate branch (13a) substantially perpendicularthereto and substantially radially in relation to the longitudinal axisof the cavity (11) and a second one o f the two lateral branches (13c)extends from a second opposed end of the central elongate branch (13a)substantially perpendicular thereto and substantially radially inrelation to the longitudinal axis of the cavity (11).
 6. The inductionheating device according to claim 5, wherein at least one of the lateralbranches extends adjacent a vicinity of a lateral wall delimiting thecavity (11).
 7. The induction heating device according to claim 6,wherein one of the lateral branches of the yoke extends radially inrelation to the longitudinal axis along a bottom surface of the ladleand the other lateral branch of the yoke being a free section directlyattached to a cover for closing the ladle and the other lateral branchextends radially relative to cover adjacent the vicinity of the lateralwall delimiting the cavity (11).
 8. The induction heating deviceaccording to claim 1, wherein each yoke is I-shaped and comprises anelongate branch (13a), which extends substantially parallel to alongitudinal axis of the cavity (11), and two lateral branches (13b,13c), a first one of the two lateral branches (13b) extends from a firstend of the central elongate branch (13a) substantially perpendicularthereto along a top surface of the cavity (11) and a second one of thetwo lateral branches (13c) extends from a second opposed end of thecentral elongate branch (13a) substantially perpendicular thereto andalong a bottom surface of the cavity (11).
 9. The induction heatingdevice according to claim 8, wherein at least the first one of thelateral branches extends radially as far as a lateral wall delimitingthe cavity.
 10. The induction heating device according to claim 2,wherein the induction coil (14) extends substantially along an entirelength of the elongate branch (13a) of the respective yoke (13).
 11. Theinduction heating device according to claim 1, wherein the inductionheating devices (13, 14), which comprise at least two magnetic yokes(13) each having at least one independent induction coil (14), areequally spaced at regular intervals about a periphery of the cavity(11).
 12. The induction heating device according to claim 11, whereinthe induction coils (14) are each fed individually by an alternatingelectric current which is phase-shifted from one induction coil (14) toanother induction coil (14).
 13. The induction heating device accordingto claim 12, wherein the alternating electric current which isphase-shifted from one induction coil (14) to another induction coil(14) is determined by an arithmetical progression.
 14. The inductionheating device according to claim 13, wherein the induction coils (14)are supplied with electrical supplied by several generators.
 15. Aninduction heating device (10, 30), for heating a metal, comprising:anoven (32) defining at least one cavity (31) for receiving and heatingbillets of a metal to be heated, via induction heating devices (33, 34),to a temperature lower than a melting point of the metal but sufficientto facilitate forging of the metal, the induction heating devices (33,34) comprising at least two magnetic yokes (33) arranged around aperiphery of the cavity (31) and having a longitudinal length which isgreater than a height of the cavit each yoke having at least oneindependent induction coil (34), and each induction coil (34) is woundin the same direction so that a north pole, of each one of the inductioncoils (34), is located at one end of the cavity (31 ) and a south pole,of each one of the induction coils (34), is located an opposite end ofthe cavity (31), each one of the induction heating devices (33, 34)generating, about the periphery of the cavity, a non null field zone(41) and a null magnetic field zone (40) being created between eachadjacent pair of non null field zones (41); each non null field zonecomprising a centrally located maximum field zone and a decreasing fieldgradient zone arranged on either side of the central maximum field zonewith each decreasing field gradient separating the central maximum fieldzone from one of the null field zones (40); and a central null fieldzone (40) located in the center of the cavity, and each one of the nonnull field zones forming an active heating zone and each one of the nullfield zones forming an inactive heating zone.
 16. The induction heatingdevice according to claim 15, wherein each of the at least two yokes(33) has a U-shaped profile and comprises a central elongate branch(33a), which extends substantially parallel to a longitudinal axis ofthe cavity (31), and two lateral branches (33b, 33c), a first one of thetwo lateral branches (33b) extends from a first end of the centralelongate branch (33a) substantially perpendicular thereto and a secondone of the two lateral branches (33c) extends from a second opposed endof the central elongate branch (33a) substantially perpendicularthereto.
 17. The induction heating device according to claim 15, whereineach yoke has a C-shaped profile and comprises an elongate branch (33a),which extends substantially parallel to a longitudinal axis of thecavity (31), and two lateral branches (33b, 33c), a first one of the twolateral branches (33b) extends from a first end of the elongate branch(33a) substantially perpendicular thereto and substantially radially inrelation to the longitudinal axis of the cavity (31) and a second one ofthe two lateral branches (33c) extends from a second opposed end of thecentral elongate branch (33a) substantially perpendicular thereto andsubstantially radially in relation to the longitudinal axis of thecavity (31).
 18. The induction heating device according to claim 16,wherein each yoke is I-shaped and comprises an elongate branch (33a),which extends substantially parallel to a longitudinal axis of thecavity (31), and two lateral branches (33b, 33c), a first one of the twolateral branches (33b) extends from a first end of the central elongatebranch (33a) substantially perpendicular thereto along a top surface ofthe cavity (31) and a second one of the two lateral branches (33c)extends from a second opposed end of the central elongate branch (33a)substantially perpendicular thereto and along a bottom surface of thecavity (31).
 19. The induction heating device according to claim 18,wherein at least the first one of the lateral branches extends as far asa vicinity of a lateral wall delimiting the cavity.
 20. An inductionheating device for heating a metal, comprising:a heating member definingat least one cavity for receiving and heating a metal to be heated, viainduction heating devices to a temperature at least sufficient tofacilitate forging of the metal, the induction heating devicescomprising at least two magnetic yokes arranged around a periphery ofthe cavity and having a longitudinal length which is greater than aheight of the cavity; each yoke having at least one independentinduction coil, and each induction coil is wound in the same directionso that a north pole, of each one of the induction coils, is located atone end of the cavity and a south pole, of each one of the inductioncoils, is located an opposite end of the cavity, each one of theinduction heating devices generating, about the periphery of the cavity,a non null field zone and a null magnetic field zone being createdbetween each adjacent pair of non null field zones; each non null fieldzone comprising a centrally located maximum field zone and a decreasingfield gradient zone arranged on either side of the central maximum fieldzone with each decreasing field gradient separating the central maximumfield zone from one of the null field zones; and a central null fieldzone located in the center of the cavity, and each one of the non nullfield zones forming an active heating zone and each one of the nullfield zones forming an inactive heating zone.